Most horn relays overheat and start to cycle on and off after a few minutes of continuous operation. How long have you had that one in service? I used to try to run 18-wheeler clearance lights on horn relays, and never got one to last more than 15 minutes or so. You need a continuous-duty relay for that application.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!